This invention is directed to apparatus and a method for delivering inert gas to the discharge passageway in a slide gate used to drain liquid metal from a metallurgical vessel, and in particular, this invention is directed to a dynamic control system that delivers argon gas at a target gas flow rate to prevent, for example but not limited to, alumina plugging in a slide gate discharge passageway that drains liquid steel from a tundish into a continuous caster.
In steelmaking operations, a slide gate is used to control the flow of liquid steel through a nozzle arrangement that drains the molten liquid steel from a metallurgical vessel. It is well known in the art that when inert gas is injected into the discharge passageway of the slide gate, the injected inert gas will reduce plugging or build-up that clogs the passageway. Continuing advancements in the art have led to the use of porous, gas permeable nozzles and slide gate plates that are able to deliver a continuous or intermittent inert gas flow to the discharge passageway where the delivered gas provides a gas barrier between the passageway surface and the liquid metal being drained. Such porous nozzles and slide gate plates are disclosed in U.S. Pat. No. 5,431,374 incorporated herein in its entirety by reference.
Referring to columns 1 and 2, the 374 patent discloses, although it is not certain, it is believed the inert gas flows through the porous nozzle walls, and advantageously forms a fluid film over the surface of the bore within the nozzle that prevents the liquid metal from making direct contact with the inner surface forming the bore. By insulating the bore surface from the liquid metal, the fluid film of gas prevents the small amounts of alumina that are present in such steel from sticking to and accumulating on the surface of the nozzle bore. The 374 reference also teaches that such alumina plugging will occur within the bore of a slide gate if an inert gas barrier is not provided. Therefore, as clearly taught in the art, for example, U.S. Pat. Nos. 4,756,452, 5,137,189, 5,284,278, and 5,431,374, inert gas barriers are used throughout the steelmaking industry to prevent alumina plugging within the discharge passageway that drains liquid steel from a tundish into the caster mold portion of a continuous caster.
Additionally, the 374 patent also discloses that in order to provide a proper inert gas barrier, the pressure of the inert gas must be maintained at a level sufficient to overcome the considerable back-pressure that the draining liquid steel product applies against the surface of the bore, and ideally, the gas pressure should be just enough to form the desired film or barrier. It is well accepted that injecting inert gas into a slide gate discharge passageway does reduce the plugging phenomenon, but metering the actual gas flow to the discharge-opening has long been a problem. Leaks in the gas delivery system are a repeating and continuous problem, and the measured amount of incoming gas flow is often different from the actual gas flow delivered to the liquid metal draining through the slide gate. Such gas delivery system leaks can occur in any one of the numerous pipefitting connections along the gas feed line extending between the inert gas supply and the slide gate mechanism. Additionally some leaks are dynamic in that they develop in the slide gate plates during casting operations as taught in U.S. Pat. No. 4,555,094. Historical information at our continuous casting operations shows that in many instances, no inert gas is delivered to the slide gate discharge passageway when the control gage readings show that the inert gas flow through the gas feed line is normal. The currently employed constant pressure or constant flow based control methods that are used to deliver inert gas to a slide gate mechanism cannot compensate for dynamic leaks, flow resistance changes, or unknown pressure drops, and therefore, they are ineffective for maintaining a target threshold gas flow within the discharge passageway. Consequently, the state-of-the-art inert gas delivery systems often fail to shield the bore surface from the liquid metal as taught in U.S. Pat. No. 5,431,374.
It is therefore an object of the present invention to provide an inert gas delivery system capable of providing a target threshold inert gas flow that prevents plugging within the discharge nozzle arrangement through which liquid metal is drained.
It is a further object of the present invention to provide an inert gas delivery system capable of measuring the amount of inert gas delivered to the discharge opening passageway where the system is capable of measuring the amount of inert gas lost through leaks so that the inert gas flow is maintained at a target threshold pressure within the discharge opening passageway.
It is another object of the present invention to provide an inert gas delivery system capable of measuring inert gas flow resistance to determine an amount of plugging that occurs within the discharge opening passageway that drains liquid metal from a metallurgical vessel.
It is an additional object of the present invention to provide a mathematical model that provides on-line evaluation and dynamic control of the inert gas delivery system so that a consistent inert gas flow is maintained to prevent or reduce plugging within the discharge opening passageway that drains liquid metal from a metallurgical vessel.
In satisfaction of the foregoing objects and advantages, the present invention provides a dynamic control system capable of delivering an inert gas at a target threshold gas flow rate to the discharge passageway in a slide gate draining a liquid metal product. The dynamic control system maintains the inert gas at a constant target threshold flow rate sufficient to prevent or reduce plugging within the discharge opening, and the dynamic control system includes a gas feed line extending between an inert gas supply and the slide gate discharge passageway, a gas flow regulator, a pressure gauge; and a gas feed flow control system that detects an amount of incoming inert gas flow lost through leaks in the system and adjusts the gas flow regulator in response to the detected amount of incoming gas flow loss so that the adjusted incoming gas flow continues to deliver the target inert gas flow rate to the discharge passageway.